In order to open any DC circuit, the inductive energy stored in the magnetic fields due to the flowing current must be absorbed; it can either be stored in capacitors or dissipated in resistors (arcs that form during opening the circuit are in this sense a special case of a resistor). Because of the rapid inrush of current in a dead short, the inductive energy can easily be much greater than just the inductive energy stored in the system at full normal load; if the current goes to double the normal full load amps before being controlled, the inductive energy would be up to four times as large as in the circuit at full load (depending on the location of the short).
Many prior art DC circuit breaker concepts rely on a preliminary commutation of the current from a low loss Switch #1 to a resistor network to dissipate the magnetic energy or a capacitor network to store the energy, or some combination of these. Switch #1 is in all cases a commutating switch, which forces the current through a parallel path through a switched network of resistors and capacitors. In the prior art, switching over multiple different paths through the circuit breaker after the initial commutation is accomplished by separate switches, with the added burden to guarantee exact synchronization of the switching events. Non-linear resistors such as metal oxide varistors (MOVs) or resistors with large positive change of resistance with increasing temperature (positive temperature coefficient “PTC” resistors or “thermistors”) have been used in various designs. Preliminary quenching or storage of most of the inductive energy prior to opening the circuit at relatively low current is especially important in HVDC circuits.
The ultimate breaking of the DC current in prior art devices (which occurs after the first commutation away from the low loss connection in the switch where such commutation occurs) relies either on:                1. quenching an arc;        2. diverting the final bit of current through an MOV or another type varistor;        3. diverting the final bit of current into a capacitor or battery for storage.        
Examples of fast switches that are used in AC/DC converters and that may also be used in DC circuit breakers include semiconductor switches such as a gate turn-off thyristor (GTO) or an integrated gate bipolar transistor (IGBT) or tube-based switches such as mercury arc valves or cold cathode vacuum tubes, all of which are known in the prior art. These switches do not by themselves comprise a circuit breaker, because the magnetic energy stored in the flowing current must be dissipated. In case of a dead short, the current increases rapidly, until the circuit breaker staunches the inrush of current by means of increasing resistance. The time it takes to cause dI/dt (the change of current with time) to go from being positive to negative is a crucial variable in circuit breakers; I shall refer to this time as the Current Change Reversal time.
Several prior art strategies are known for breaking a high power DC current. Arc chute breakers (U.S. Pat. Nos. 2,270,723; 3,735,074; 7,521,625; 7,541,902 for example) are effective to break DC currents up to 8000 amps at 800 volts (0.8 kV) DC, or 4000 amps at 1600 volts (1.6 kV). One can go to higher voltage in principle with arc chute breakers, but the needed physical separation of the electrodes increases linearly with voltage in such devices, and so they become impractically large at voltage higher than 3.5 kV. One can also go to higher voltage with arc chute breakers by putting two or more arc chute breakers in series, and opening all of them simultaneously. Arc chute breakers can be made more effective by judicious use of magnetic fields, which may be applied either by permanent magnets or electromagnets, or both. Advanced materials are used both for the electrodes and for the surfaces of the arc chutes, to minimize damage caused by the arcs.
The concept behind arc chute breakers is to spread out the arc over a large surface area. Since the arc is quite hot, the higher surface area implies far greater radiative cooling. As the arc cools, it is also elongated; the resistance goes up so high that the arc is ultimately quenched; this process takes a while: 300 milliseconds (ms) is a typical time between striking the arc and arc extinction in an arc chute breaker. This long time to open the circuit has little to do with the speed of motion of the electrodes; in a Gerapid™ circuit breaker from GE, for example, the electrodes are separated within 3 ms (milliseconds), but cooling the arc takes about 100 times as long as that, and the current can continue to increase in case of a dead short for tens of ms in an arc chute circuit breaker before the current inrush due to the dead short is reversed towards zero current. Because of the long time that it takes to extinguish the arc, a lot of energy (far more than just the stored magnetic energy in the circuit at full load) must be dissipated into the arc chutes, which get quite hot. One way to prevent melting of the arc chutes is to arrange a circuit (as in U.S. Pat. No. 3,566,197 for example) that moves the arc from one arc path to the next in such a way as to allow the individual arc paths to cool between periods of use, until the arc is quenched. (Note, though, that the specific design of U.S. Pat. No. 3,566,197 will only work for AC current.)
Another means known in the prior art to create a high power DC circuit breaker is to use the charging or discharging of a capacitor to momentarily reduce the voltage and current to a level that a fast acting AC-type switch can open the circuit. U.S. Pat. No. 3,809,959 describes an arrangement in which two AC-type switches, a resistor, a spark gap, and a capacitor are combined to give an effective DC circuit breaker that can work up to HVDC voltage. Referring to U.S. Pat. No. 3,809,959, a fast Switch #1 opens and commutates most of the current to a single Resistor #4. Depending on the current flowing, Switch #1 may still have an arc between the contacts after diverting most of the current to the resistor. The insertion of the resistor and spark gap through Switch #1 causes the voltage to climb enough to jump over Spark Gap #3 to Capacitor #2. During the period of charging this capacitor, the current goes to nearly zero in the path through the arc in the second AC-type switch, Switch #5 and the arc is extinguished, which opens the circuit. This is faster than an arc chute breaker, and is applicable up to HVDC voltage levels with a reasonably compact design. Later refinements of this idea include pre-charging the capacitor to an opposite polarity compared to the flowing current to be interrupted, so that the voltage is momentarily reversed in Switch #5, forcing the arc there to go through zero current and zero voltage (which increases the chance to interrupt the current). Another known refinement is to use a thermistor for Resistor #4. The device of U.S. Pat. No. 3,809,959 is still used in HVDC AC/DC converter stations to allow for fast isolation of one of the two (+) and (−) poles of the HVDC system in case of a ground fault on one leg of an HVDC bipole system (This type of breaker is called a “metallic return transfer breaker”). In this case, half of the HVDC system can be quickly isolated from the opposite pole, which allows temporary use of one pole with ground return (or metallic return through a low voltage conductor) while the other pole is fixed.
U.S. Pat. No. 3,534,226 describes a particular way to insert resistance and capacitance into a DC circuit, to open the circuit; this patent is included herein by reference in its entirety. The basic concept of switching in resistors to reduce the current in a stepwise manner so as to control the magnitude of voltage transients during opening of a DC circuit is well described in U.S. Pat. No. 3,534,226, which envisions using many individual switches and resistors. The method of U.S. Pat. No. 3,534,226 involves two different kinds of switches that must be opened in a precise sequence: first a low resistance mechanical switch (through which most of the power flows when the circuit breaker is closed) is opened. This is a conventional switch in which the electrical contacts are separated. Although a plasma arc may briefly form between the separating electrodes of the low resistance switch, this arc is quickly extinguished as the current is commutated onto a parallel path through the resistors, which are switched via fast acting switches; these fast acting switches can be mercury arc valves or other types of fast switching tubes, or solid state devices like IGBTs or GTOs that can accomplish switching within 10 microseconds. By the time the last fast acting switch is opened the current has been reduced to less than 10% of its maximum value (which implies that >99% of the magnetic energy has been dissipated), which allows the final capacitor snubber to be relatively small and economical compared to the size it would have to be if it had to absorb most of the magnetic energy stored in the circuit at the time of initial opening. U.S. Pat. No. 3,534,226 forms the basis for several subsequent patents, including U.S. Pat. Nos. 3,611,031 and 3,660,723 (both of which also use a low-loss mechanical switch to commutate the current to a resistive network based on fast electronic switches), and U.S. Pat. No. 6,075,684 which uses a fast electronic switch in place of the commutating mechanical switch.
U.S. Pat. No. 3,777,178 describes a particular way to insert resistance and capacitance into a DC circuit. This design uses at least three switches (D1 and D2 are commutating switches, while D3 is the switch that accomplishes the final opening of the circuit), two capacitors, and two resistors (which are preferably varistors); the switches themselves are not described in detail, but are presumably of prior art designs such as arc chute breakers, gas blast breakers, vacuum circuit breakers, or SF6 gas-insulated switchgear. In the end, the final part of the inductively stored energy must be stored in the capacitors after switch D3 opens.
U.S. Pat. No. 3,777,179 (Hughes Aircraft) describes a particular way to insert resistance and capacitance into a DC circuit.
U.S. Pat. No. 4,300,181(GE) describes a means of breaking a DC current by using a capacitor of minimum size that is charged up prior to breaking the circuit. This circuit breaker design utilizes varistors to absorb the inductive energy that must either be stored or dissipated on opening the circuit.
Several designs of resonant DC circuit breakers are known, for example U.S. Pat. Nos. 4,216,513 and 4,805,062 (Hitachi) and US patent application 2011/0175460 (ABB). These devices create an L-C oscillation (an inductor-capacitor oscillation) that is superimposed on the DC current by placing inductors and capacitors in series connection in such a way that an exponentially decaying “ringing” of the circuit occurs when the capacitor is discharged into the circuit. The ringing of the circuit should ideally have high enough amplitude that the current and voltage cross zero in the first few oscillations. This allows an AC-type circuit breaker to open the circuit. US patent application 2011/0175460 is a particularly elegant configuration, resulting in a fairly compact HVDC circuit breaker which oscillates through zero voltage and current several times during its ringing decay; this ABB patent shows a range of inductance and capacitance where a particular current may be broken by the AC type circuit breaker that is opened to initiate the ringing decay response.
U.S. Pat. No. 6,501,635 describes a particularly fast acting mechanical switch in which a conductive ring has an induced current that interacts with the magnetic field of a stationary electromagnet so that the ring is strongly repelled and therefore moves quite fast (in about one ms) from a closed to an open position within the switch. Such a switch can be used in AC circuit breakers that wait for the next zero crossing to break the circuit. Because the mass of the ring is much less than the mass of all the parts that typically must move when a mechanical circuit breaker opens, this “electrodynamic ring breaker” is fast for a mechanical switch. This switch by itself is not useful as a high power DC circuit breaker; however, it can be combined with a switched array of resistors as in U.S. Pat. No. 3,534,226 to create a DC circuit breaker. A paper by Michael Steurer, Klaus Fröhlich, Walter Holaus, and Kurt Kaltenegger: “A Novel Hybrid Current-Limiting Circuit Breaker for Medium Voltage: Principle and Test Results,” IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 18, NO. 2, APRIL 2003 describes a hybrid MVDC circuit breaker based on a similar principle to that of U.S. Pat. No. 3,534,226, except for using a single thermistor rather than a switched array of resistors to clamp down on the surging current in a short. A problem with this design is that the current has to be high enough to heat up the thermistor for the proposed mechanism of Steurer et al to work properly.